Ryanodine is a Positive Modulator of Acetylcholine Receptor Gating in Cochlear Hair Cells

JAVIER ZORRILLA DE SAN MARTÍN; JIMENA BALLESTERO; ELEONORA KATZ; ANA BELÉN ELGOYHEN; PAUL FUCHS

Denver, Colorado, USA

Congreso; Thirtieth annual midwinter research meeting of the Association for research in otolaryngology; 2007

Association for research in otolaryngology (ARO)

The efferent synaptic specialization of hair cells includes a near-membrane synaptic cistern whose presence suggests a role for internal calcium stores in cholinergic inhibition. Calcium release channels from internal stores include ‘ryanodine receptors’ whose participation is usually demonstrated by sensitivity to the eponymous plant alkaloid, ryanodine. However, use of this and other storeactive compounds on hair cells  is confounded by the unusual pharmacology of the alpha9alpha10-containing hair cell ACh  receptor (AChR), which has been shown to be antagonized by a broad spectrum of  compounds. Surprisingly, we found that ryanodine is a positive modulator of the hair cell  AChR, the first such compound to be found. ACh-evoked currents through the isolated cholinergic receptor of inner hair cells (IHCs) in excised mouse cochleas were  enhanced ~ 50% by 200 μM ryanodine, a concentration that inhibits gating of the  ryanodine receptor itself. This facilitatory action appears to be on the AChR itself, since  a similar effect was seen for ACh-evoked currents in Xenopus oocytes injected with rat alpha9 and alpha10 subunit mRNAs, and corresponds to an increase in the apparent affinity for ACh. This unusual positive modulation is not unique to the mammalian receptor. The response of chicken hair cells exposed to ACh is likewise enhanced in the presence of  100 μM ryanodine. This effect can be seen both for currents through AChRs alone,  isolated by strong internal calcium buffering; or when the associated SK currents are present in hair cells with weaker calcium buffering. In this latter case, the SK currents become longer-lasting. Finally, the action of ryanodine directly on the hair cell ACh  receptor is further supported by the observation of similar effects in hair cells from the crooked neck dwarf chicken, a naturally occurring mutant with defective ryanodine receptors. This novel effect of ryanodine provides new opportunities for design of inner ear therapeutics based on this interaction.